Electron discharge devices with a sharp edged cathode



Oct. 29, 1963 P. 1.. SPENCER 3 0 23 ELECTRON DISCHARGE DEVICES WITH A SHARP EDGED CATHODE 2 Sheets-Sheet 1 Filed March 15, 1962 Il IIIIIII/II- I/ll/Ill/IAI F/GJ I/WENIUR F/G. Z PERCY L. SPENCER Oct. 29, 1963 ,P. SPENCER ELECTRON DISCHARGE DEVICES WITH A SHARP EDGED CATHODE Filed March 15, 1962 2 Sheets-Sheet 2 INVENTOR PERCY L. SPENCER TQM AGE/VT United States Patent 3,109,123 ELECTRON DISCHARGE DEVICES WITH A SHARP EDGE!) CATHQDE Percy L. Spencer, Wahan, Mass, assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Mar. 15, 1962, Ser. No. 179,853 12 Claims. (Cl. 31539.63)

This invention relates to electron discharge devices having a substantially continuous cathode bounding at least a portion of a waveinteraction space, and more particularly to a cathode structure for use in such devices whereby electron flow commences without the requirement of first heating the electron emissive material of the cathode with energy from an external source.

Heretofore, cold cathodes have been employed in magnetron type electron discharge devices. More particularly, they have been employed in amplitrons which must be started in a prescribed manner whereby the electron emissive material of the cold cathode is bombarded initially to produce substantial numbers of electrons to commence operation of the device. An amplitron including a cold cathode is described in copending application Serial No. 803,326 now Patent No. 3,096,457, entitled Traveling Wave Tubes, by William A. Smith, in, et al. The copending application describes a structure and a suitable method for initiating operation of the amplitron. For example, operation is initiated by first producing a unidirectional transverse magnetic field in the interaction region of the amplitron, then supplying high frequency wave energy to the input of the amplitron, and finally producing a unidirectional transverse electric field in the interaction space. The high frequency wave energy ionizes gas molecules in the interaction space so that when the electric field is produced therein, positive gas ions are driven toward the 'cold cathode, strike the emissive material and cause emission of an abundance of secondary electrons which, in turn, proceed through the interaction space. These electrons interact with the high frequency wave energy conducted by the anode and impart energy to the waves amplifying them. It is important in this operation that the high frequency wave energy be turned on and applied before the transverse electric fiel so that a substantial number of gas molecules are ionized throughout the interaction space before the electric field is turned on. If the electric field were turned on first, the gas ions would be removed as quickly as they are formed, and it would become difiicult to build up a concentration of secondary electrons at any given instant sufiicient to initiate operation. This feature of the prior system obviously imposes some restrictions on the procedure for starting. It is one object of the present invention to provide such a magnetron type device wherein operation can be initiated without first ionizing gas molecules by the application of an externally generated high frequency signal.

Another and more specific object of the invention is to provide a continuous cathode for a magnetron device in which the electron emissive portion of the cathode is so contoured to insure easy starting and require application of little or no heat from an external source to initiate electron emission from the material.

In accordance with the present invention, the electron emitting surface of the cathode of a magnetron type device includes at least one sharp edge and is preferably disc shaped, the sharp edge of the disc lying in a plane transverse to the axis of the device. As a result of this construction, the transverse electric field in the interaction space normally bounded by the anode (Wave propagating structure) and the cathode is concentrated to a high intensity in the immediate vicinity of the sharp edge so 3,169,123 Patented Oct. 29, 1963 that when this electric field is turned on, small incipient arcs occur at the sharp edge, and these arcs ionize gas molecules. The negative ions thus formed strike the electron emissive material in the vicinity of the sharp edge knocking off secondary electrons and producing a flow of electrons therefrom. The electrons exchange energy with waves conducted by the structure which are either generated initially as in a magnetron or are launched into the structure from an external source as in an amplitron.

In various embodiments of the invention the oathode includes one or more sharp edges, and shielding structures are provided which cause a greater concentration of the transverse electric field at the sharp edges. One embodiment of the invention includes minute wires of electrically conductive material disposed along the sharp edge and generally projecting toward the anode. These wires are preferably composed of a material having a nigh melting temperature such as tungsten'and are preferably only a few thousandths of an inch in diameter. If they are composed of tungsten, the wires are preferably plated at their exposed ends to reduce the tendency which materials such as tungsten have to catch and absorb gas ions and molecules (gettering). Accordingly, a plating of refractory material such as rhenium, iridium or platinum may be used. Other features and objects of the invention will be more apparent from the following specific description of embodiments of the invention taken in conjunction with the figures in which:

FIG. 1 illustrates a side-sectional view of a magnetron type device including a cathode incorporating features of the invention;

FIG. 2 illustrates a plan-sectional view of the magnetron ty-pe'devicej FIG. 3 illustrates a view of the device to show the assembly of the envelope with the permanent magnet;

FIGS. 4 and 5 show enlarged plan and side views of the electron emitting portion of the cathode with minute wires of conductive material fastened along the sharp edge thereof; and

FIGS. 6, 7 and 8 illustrate additional embodiments of the invention each including electric field shields for concentrating the electric field in the immediate vicinity of the sharp edges of the cathode.

Turning first to FIG. 1, there is shown a side-sectional view of a magnetron type electron discharge device which may be, for example, a magnetron, an amplitron, a stabilitron or other type of electron discharge device including a continuous cathode generally concentric with a wave propagating structure bounding an interaction space thereb etween in which electrons interact and exchange energy with waves either sustained or propagating in the structure. The particular device illustrated in FIG. 1 is a magnetron. The envelope 1 of the magnetron includes upper and lower plates 2 and 3 disposed concentric with the magnetron axis 4, the plates having annular openings 5 and 6 to accommodate portions of the cathode 7. The upper and lower plates are sealed to anode ring 8 to which is attached a wave sustaining or propagating structure 9. The structure 9 may include a plurality of wave sustaining cavities as ordinarily associated with a magnetron anode or it may form a slow wave propagating structure such as normally associated with the anode in an amplitron. In either case, the structure 9 serves to conduct high frequency wave energy so that the electric and magnetic fields of the wave energy couple to the interaction space 10 which is the annular space between the structure 9 and the cathode 7. The output of the device is obtained by coupling a transmission line such as waveguide 11 to one of the lumped circuits or cavity 12 of the structure 9. The coupling may be accomplished as shown 3. through an opening 13 in the anode cylnder 8, the opening being mechanically sealed by a window 14 which is substantially transparent to the wave energy.

The structure 9 illustrated is formed of a plurality of conductive vanes such as 15 and 16 disposed radially with respect to the axis 4 and attached to the inner wall of anode cylinder 8. These vanes as such define a plurality of wave sustaining cavities disposed about and concentric with the axis 4. In addition, however, pairs of conductive straps such as 17 and 18 are provided, each strap of each pair being attached to a ditferent group of alternate vanes. One theory of operation is that by virtue of the strapping of alternate vanes, waves sustained in adjacent cavities are in opposite phase. An other theory is that each pair of straps performs as a two-element transmission line across which are numerous lumped circuits along the length of the line, the lumped circuits being formed by the vanes. The present invention contemplates a cathode structure compatible with either theory and useful in unstrapped magnetrons, strapped magnetrons, amplitrons, stabilitrons, and, in fact, any crossed-field type electron discharge device employing a cathode bounding a substantial portion of the interaction space.

The cathode shown in FIGS. 1 and 2 includes a support stem 21 concentric with the axis 4 with a disc-shaped electrically conductive member 22 disposed concentric therewith and attached to the stem. The edge 23 of the disc is preferably sharp and is coated with an electron emissive material 24, this edge being disposed in a plane transverse to the tube axis and substantially bisecting the vanes. As shown, each end of the stem 21 is supported by a structure fastened to the upper and lower plates 2 and 3. The stem, however, could be supported in a cantilevered manner from only one of the plates. The upper end of the stem electrically connects to a terminal 25 which is insulatedly supported from the upper plate. This support might, for example, consist of a cylinder of conductive material 26 which seals to the opening in upper plate 2 and attaches rigidly to an insulator 27 which in turn supports the terminal 25, the complete structure serving to support the stem 5 and to insulate it from the envelope of the magnetron. The

other end of the stem is slidably supported by an electrically insulating sleeve 28 which is in turn rigidly supported within cylinder 29 which seals to opening 6 in the lower plate 3. The end of this support is sealed by a cap 30. Thus, the cathode stem 21 is supported within the sealed envelope of the magnetron at both its ends. One end is fixedly supported and electrically connects through the envelope to a terminal 25, and the other end is slidably supported to permit expansion along the axis of the tube.

In operation, a transverse magnetic field is produced in the interaction space It) by magnets 31 and 32. The magnets are preferably U-shaped as shown in FIG. 3 and arranged with their like poles abutting as shown in FIG. 3 producing a substantially uniform transverse magnetic field throughout the interaction space 10. The anode and envelope are preferably at ground potential so that when a negative potential is applied to the cathode stem 5, a transverse electric field is produced in the interaction space 10, and the magnetron commences to operate producing a radio frequency output at the waveguide 11. The tube commences operation when the electric field is turned on producing the high electric field intensity and ionization of gas molecules in the vicinity of the sharp edge 23 as already described above. Small arcs will occur in such a field even if the ion concentration in the residual gas within the envelope is as low as mm. of Hg, and these arcs will in turn ionize more of the gas molecules. The positively charged ionized gas molecules thus produced are driven by the transverse electric and magnetic fields to bombard the emissive surface of the cathode, this bombardment being particularly high in the immediate vicinity of the sharp edge. Such bombardment knocks secondary electrons from the cathode, and these secondary electrons supplemented by electrons produced by the ionization provide the necessary electron concentration in the interaction space 10 to initiate tube operation as in a magnetron oscillator. Thereafter, the cathode temperature and electron emission therefrom are maintained by the same sort of ionization process. In addition, the cathode is further heated by back bombardment of electrons which interact with the radio frequency waves sustained or propagating in structure 9 and gain sufiicient energy therefrom to bombard the cathode. The theory of back bombardment by electrons in such a device is well known and requires interaction between the electrons and the fields of the radio frequency waves. However, back bombardment cannot itself commence until waves are first induced or caused to propagate in the structure. The purpose of creating the incipient arcs is to increase the concentration of molecular ions which strike the cathode knocking off electrons to provide an initial concentration of electrons which are compelled by the transverse electric and magnetic fields to move along cycloidal-type paths, thereby inducing the waves. After this occurs, then the back bombardment by electrons occurs.

FIGS. 4 and 5 illustrate an embodiment of the invention showing a cathode structure similar to the cathode structure 7 in FIGS. 1 and 2. The electron emissive surface 41 of the cathode is disc shaped, and the bulk of the cathode 42 beneath this surface is preferably composed of a highly thermally conductive material of high density so that it will absorb and hold a substantial amount of heat and tend to provide a uniform flow of heat to the electron emissive material to insure a uniform flow of electrons therefrom during operation. The cathode body 42 is supported on a stem 43 which in turn is supported in a manner similar to that shown in FIGS. 1 and 2. In addition, a multitude of minute wires 44 are provided along the edge 45 of the emissive surface. Each of these wires tends to gather electric field and thus concentrate the electric field in points of very high intensity, thereby insuring a high degree of molecular ionization.

The wires are a few thousandths of an inch or less in diameter and are preferably of tungsten or some other high melting temperature metal. If they are of tungsten or a similar metal which has a tendency to capture and absorb gas molecules, then they are preferably coated with some material which does not have such a tendency and which need not be highly electrically conductive, but which must also have a relatively high melting temperature. Accordingly, the tungsten wires may be coated with rhenium, iridium or platinum or materials which generally fall in the classification of refractory.

In operation the electric field intensity in the interaction space 10 is sustained between the structure 9 and the cathode and is of relatively low intensity immediately adjacent each of the wires 44 which are disposed along the sharp edge 45 of the cathode. In view of the general shape of this electric field represented by lines 46, back bombardment of the cathode during operation will be most intense in the immediate vicinity of the sharp edge 45 and will be practically negligible along the surface 47 of the cathode. In order to insure a more uniform distribution of temperature throughout the bulk of the cathode, the cathode body is preferably composed of a number of layers of material having different heat capacities and thermal conductivities. For example, as shown in FIG. 5, the cathode body is composed of three layers 48, 49 and S1. The center layer 48 is preferably more highly thermally conductive than the layers 49 and 51. However, the layers 49 and 51 are preferably of higher density and/ or specific heat. As a result, heat generated by back bombardment in the immediate vicinity of the sharp edge 45 will flow rapidly from layer 48 into thetwo adjacent layers 49 and 51 and will be stored in the layers 49 and 51. Thus, the layers 49 and 51 act as a thermal reservoir to prevent 48 from becoming too hot during instants of high back bombardment and to prevent it from cooling excessively during instants of low back bombardment.

Turning next to FIGS. 6, 7 and 8 there are shown additional embodiments of the invention each including one or more generally disc-shaped cathode bodies with a sharp edge facing the wave propagating structure 9, each cathode being supported on a stem 61 in substantially the same manner as shown in FIGS. 1 and 2. In addition, each of these cathode structures includes electric field shields which may be energized at the same potential as the cathode as shown in FIGS. 6 and 7 or at a different potential from the cathode as shown in FIG. 8. These electric field shields are preferably flat, circular and disposed coaxially with the tube axis 4 as shown. They serve to distort the electric field in the interaction space in such a manner as to increase the concentration of the electric field in the immediate vicinity of the sharp edge of the cathode. To accomplish this, the edge of each of the shields shown in FIGS. 6 and 7 are rounded. For example, the shields 62 and 63 in FIG. 6 are made bulky toward their edges to increase the shield thickness at the edge. The same effect is accomplished with the shields 64 and 65 in FIG. 7 by merely tilting the edge of the shields away from the interaction space 10 so that the shields offer a broader surface facing structure 9. In addition, the shields in both FIGS. 6 and 7 shield the electron emissive surface of the cathode from all parts of the anode envelope such as upper and lower plates 2 and 3, and thus provide a termination for electric field lines running therefrom which would ordinarily terminate on the emissive surface of the cathode, were it not for the shields.

In FIG. 8, the shields 66 and 67 couple to voltage sources external of the tube envelope and are each preferably maintained at a potential positive with respect to the cathode as shown in the figure. Since the whole structure is symmetrical as shown with respect to a plane transverse to the tube axis and passing through the sharp edge of the cathode, the shields 66 and 67 are preferably maintained at the same potential.

The purpose of the shields in the embodiments shown in FIGS. 6-8 is generally to aid in focusing the electric field in the interaction space so that electric field running from the structure 9 to the cathode will tend to concentrate as much as possible along the sharp edge of the cathode, and while a few shielding structures are shown in FIGS. 6-8, it should be noted that numerous other types of shielding structures and formations known in the art could be substituted without deviating from the spirit or scope of the invention. In addition, it should be noted that embodiments of the invention described herein are included in a magnetron type electron discharge device. However, as pointed out, the invention is also applicable to other types electron discharge devices which include a cathode surface bounding at least a portion of the interaction space. Accordingly, the scope of the invention is as set forth in the accompanying claims.

What is claimed is: 1. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and means producing transverse electric and magnetic fields in said space, said cathode structure comprising:

a plurality of surfaces of electron emissive material,

said surfaces being joined to form a sharp edge;

and means supporting said surfaces so that said sharp edge bounds at least a part of said transverse electric field.

2. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and means producing transverse electric and magnetic fields in said space, said cathode structure comprising:

a plurality of surfaces of electron emissive material,

said surfaces being joined to form a sharp edge coextensive with said anode structure;

and means supporting said surfaces so that said sharp edge bounds at least a part of said transverse electric field.

3. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure coextensive with at least a portion of said anode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and'means producing transverse electric and magnetic fields in said space, said cathode structure comprising:

a plurality of surfaces of electron emissive material,

said surfaces being joined to form a sharp edge facing said anode structure;

and means supporting said surfaces so that said sharp edge is coextensive with said anode structure and bounds at least a substantial part of said transverse electric field.

4. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and means producing transverse electric and magnetic fields in said space, said cathode structure comprising:

a disc-shaped body of substantially thermally conductive material;

electron emissive material covering at least the edge of said disc-shaped body;

and means supporting said body so that said edge bounds a part of said transverse electric field.

5. An electron discharge device including an anode structure for conducting radio frequency Waves, a cathode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and [means producing transverse electric and magnetic fields in said space, said cathode structure comprising:

a disc-shaped body of substantially thermally conductive material; electron emissive material covering at least the edge of said disc-shaped body;

and means supporting said body so that said edge is coextensive with said anode structure and bounds a part of said transverse electric field.

6. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure coextensive at least a portion of said anode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and means producing transverse electric and magnetic fields in said space, said cathode structure comprising: g

a disc-shaped body formed of layers of materials having different thermal characteristics;

electron emissive material covering at least the edge of said disc-shaped body;

and means for supporting said body so that said edge goluinds a substantial part of said transverse electric 7. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure defining an interaction space, means for producing transverse electric and magnetic fields in said space for compelling electrons emitted from the cathode structure to interact with the fields of said waves, said cathode structure comprising:

a plurality of electron emitting surfaces joined to form a sharp edge and a multitude of minute wires disposed along said edge-projecting toward said interaction space and bounding a part of said transverse electric field.

8. An electron discharge device including an anode structure for conducting radio frequency waves and a cathode structure defining an interaction space, means for producing electric and magnetic fields in said space for compelling electrons emitted from the cathode structure to interact with the fields of said waves, said cathode structure comprising:

a plurality of electron emitting surfaces joined to form a sharp edge coextensive with said anode structure and a multitude of minute wires disposed along said edge projecting toward said interaction space and bounding a part of said transverse electric field.

:9. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure coextensive with at least a portion of said anode structure defining an interaction space, and means producing transverse electric and magnetic fields for compelling electrons emitted from the cathode structure to interact with the fields of said waves, said cathode structure comprising:

a plurality of electron emitting surfaces joined to form a sharp edge and a multitude of minute tungsten wires disposed along said edge projecting toward said interaction space for bounding a substantial part of said transverse electric field.

10. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and means producing transverse electrio and magnetic fields in said space, said cathode structure comprising:

a disc-shaped body contoured so that a substantial part of the edge of said disc defines a sharp point; electron emissive material coating at least the edge of said body;

means supporting said body and providing electrical connection thereto;

and electric field shields disposed on each side of said body for distorting said electric field to insure that a substantial part thereof is bounded by said edge.

'1 1. An electron discharge device including an anode structure for conducting radio frequency waves, a cathode structure defining an interaction space in which electrons emitted from the cathode structure interact with the fields of said waves, and means producing transverse electric and magnetic fields in said space, said cathode structure comprisin a disc-shaped body contoured so that a substantial part of the edge of said disc defines a sharp point in a plane parallel to said transverse electric field;

electron emissive material coating at least the edge of said body;

means supporting said body and providing electrical connection thereto;

and electric field shields disposed on each side of said body for distorting said electric field so that a substantial part thereof is bounded by said edge.

12. An electron discharge device including a generally cylindrical anode structure for conducting radio frequency Waves, a cathode structure coextensive with at least a portion of said anode'structure defining an interaction space, and means producing transverse electric and magnetic fields in said space for compelling electrons therein to interactwith the fields of said waves, said cathode structure comprising:

a disc-shaped body concentric with said anode structure,

a substantial part of the edge of said disc defining a sharp point in a plane transverse to the axis thereof;

electron emissive material coating at least the edge of said body; 1

means supporting said body and providing electrical connection thereto;

electric field shields disposed on each side of said body;

and means energizing said shields so that a substantial part of said electric field is bounded by said anode and said sharp edge.

No references cited. 

1. AN ELECTRON DISCHARGE DEVICE INCLUDING AN ANODE STRUCTURE FOR CONDUCTING RADIO FREQUENCY WAVES, A CATHODE STRUCTURE DEFINING AN INTERACTION SPACE IN WHICH ELECTRONS EMITTED FROM THE CATHODE STRUCTURE INTERACT WITH THE FIELDS OF SAID WAVES, AND MEANS PRODUCING TRANSVERSE ELECTRIC AND MAGNETIC FIELDS IN SAID SPACE, SAID CATHODE STRUCTURE COMPRISING: A PLURALITY OF SURFACE OF ELECTRON EMISSIVE MATERIAL, SAID SURFACES BEING JOINED TO FORM A SHARP EDGE; 